Preheat oven to 350 degrees F (175 degrees C). Grease 12 muffin cups or line with paper liners.
Mix water and chia seeds together in a small bowl; set aside until chia seed gel is thickened, about 10 minutes.
Combine flour, coconut sugar, baking soda, baking powder, and salt together in a bowl. Whisk chia gel, yogurt, oil, egg, and vanilla extract together in a separate bowl; stir into flour mixture just until batter is moistened. Fold bananas and walnuts into the batter. Fill muffin cups 2/3-full with batter.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 22 to 25 minutes. Cool muffins for 5 minutes in the tin before transferring to a wire rack to cool completely.